From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Society of Automotive Engineers preprint paper on the idiosyncrasies and causes of issues with valve mechanisms.
| Identifier | ExFiles\Box 56\2\ Scan079 | |
| Date | 15th January 1929 | |
| Handwritten: To Mr Robotham T. P. H.{Arthur M. Hanbury - Head Complaints} getter PREPRINT.—Paper to be presented at the Annual Meeting of the Society of Automotive Engineers, Detroit, Jan. 15-18, 1929. Subject to revision. This preprint is issued primarily to stimulate written and oral discussion. The paper shall not be published in full prior to Jan. 17. Idiosyncrasies of Valve Mechanisms and Their Causes By FERDINAND JEHLE¹ and W. R.{Sir Henry Royce} SPILLER² ANNUAL MEETING PAPER ILLUSTRATED WITH PHOTOGRAPHS, DIAGRAMS AND CHARTS [Left Page Text - Page 1] THE effect on engine performance of the misbehavior of the valve mechanism is threefold, resulting in loss of power, breakage and wear of parts, and noise. The nature and extent of all three effects depend only on how closely the valve follows the lift diagram laid out for it. This diagram is assumed to be correct; that is, its area is large enough and the timing is such as to result in the desired engine performance. Any discussion of valve timing and valve areas is, therefore, not within the scope of this paper. Valve bouncing will affect power in several ways: It can bring about a late closure, or even a reopening, of the exhaust-valve, which causes a serious overlapping with the inlet; it can bring about a late closure or a reopening of the inlet-valve, which will result in some of the charge being backed out; or the bounce may occur while the valve is supposed to be open, in which case it increases the opening area and will not be detrimental to the power. Breakage and wear of parts can be caused in two ways: by bouncing of the valve and by surging of the spring. Bouncing usually causes high valve-closing velocity and heavy impact on the seat, resulting in seat and valve wear. If the bounce occurs while the valve is open, the entire valve-operating mechanism may be overloaded. Spring surge usually results in overstressing the spring and increasing enormously the number of stress cycles, with a consequent fatigue break of the spring, followed perhaps by breakage of other parts. ¹M.S.A.E.—Research engineer, White Motor Co., Cleveland. ²M.S.A.E.—Laboratory engineer, White Motor Co., Cleveland. 1 [Right Page Text - Abstract & Page 4] [Column 1 - Abstract] AFTER mentioning the detrimental effects of valve bouncing and valve-spring surge upon the power and durability of an engine and on noise, the authors list four factors that contribute to perfect action of the valve mechanism. These are: the spring forces, as related to the speed and weight of the moving parts; the rigidity of the parts; the cam contour; and the design of the spring. Four different methods of investigating valve behavior are then described in detail. The telescopic point-by-point indicator and the stroboscopic projector of the valve motion were the first of these to be developed. The former gives an accurate measure of the valve position at any point of the cycle, and the latter makes possible a visual inspection of the valve operation. These two instruments were used together, but they were found to be rather slow in operation. The valve-lift-curve indicator, which was developed next, gives a photographic record of the valve-lift curve; and this was supplemented by a spring-vibration indicator which makes a record of the actual vibration of the spring on the same film with the valve-lift curve. Spring surge is the direct result of resonance of the spring frequency with the harmonics of the valvelift curve, according to the theory of the authors, which is checked experimentally. A formula, which includes both the harmonic analysis of the valve-lift curve and the characteristics of the spring, is given to aid in the selection of a good combination of spring and cam. Another formula is given for calculating the frequency of the spring in terms of its dimensions. An outline for the procedure of selecting a good combination of spring and cam concludes the paper, the suggested order being: (a) harmonic analysis of the valve-lift curve; (b) determination of the spring frequency required to avoid resonance with bad harmonics; and (c) selection of the spring with reference to the limitations. In an appendix are given a sample calculation of the harmonics of the cam and the mathematical derivation of the general equation for spring-vibration amplitude, as used in the paper. [Column 2 - Page 4] Noise, the least important result, is also caused by bouncing of the valve and surging of the spring. The former produces valve clatter, and the latter produces spring hum and coil clash. CAUSES OF FAULTY ACTION If a valve mechanism does not follow the lift curve, it is because the different factors having to do with valve performance are not properly balanced. These factors are as follows: (1) The spring force, as related to speed and weight of the moving parts (2) Rigidity of the parts (3) Cam contours (4) Design of the spring The spring must be calculated to exert a force equal to that of the negative acceleration plus the friction of the mechanism at the maximum speed. This presents no special difficulty. The different parts entering into the valve mechanism, such as cam followers, push-rods and rocker-arms, must be so designed that they will possess the maximum rigidity with the minimum weight. This also is not difficult, but rigidity should never be sacrificed for lightness. Increased weight can be met with a stronger spring, but there is no such easy remedy for errors due to flimsiness of the parts. Both the cam contour and the design of the spring are of great importance, and these will be dealt with later in the paper. Study of valve behavior through its effect on engine performance is a long, tedious, cut-and-try method. 4 | ||